The present invention relates to an improved method for measuring the pulmonary functional residual capacity (FRC) of a subject. Functional residual capacity (FRC) is the gas volume remaining in the lungs after unforced expiration or exhalation.
Two methods are currently used to measure functional residual capacity. In one such method, the subject is positioned in a gas tight body box. The subject's airways are sealingly connected to a breathing conduit connected to the exterior of the body box. The conduit has a valve or other means for temporarily closing the conduit. By measuring lung pressures and pressures in the box, at various respiration states and valve conditions, the functional residual capacity can be determined.
However, this method is not suitable for critically ill and/or artificially ventilated patients because spontaneous breathing action and movement of the patient into and out of the body box are required.
The inert gas wash-out measurement technique for functional residual capacity is based on a determination of the amount of gas exhaled from the subject's lungs and corresponding changes in gas concentrations in the exhaled gas. The gas used for the measurement is inert in the sense that it is not consumed during respiration. This gas is also sometimes called a "marker," "tracer," or "indicator" gas. Typical gases used for functional residual capacity measurements are nitrogen, helium, argon, and sulfur hexafluorine (SF.sub.6). Nitrogen is used to describe the technique below.
Initially, the lung volume forming the functional residual capacity contains nitrogen in the same percentage as in air, i.e. approximately 80%. In the wash-out measurement technique, the subject commences breathing pure oxygen. With each breath, nitrogen in the lungs is replaced by oxygen, or, stated conversely, the nitrogen is "washed out" of the lungs by the oxygen. The breathing of pure oxygen could continue until all nitrogen is washed out of the lungs. In practical uses of the technique, the breathing of pure oxygen continues until the nitrogen concentration in the exhaled respiratory gases falls below a given concentration.
The gist of the inert gas wash-out technique is to determine the volume of inert or indicator gas washed out of the lungs, and, knowing the initial concentration of indicator gas in the lungs, to compute the functional residual capacity of the lungs from these quantities. In the case of air and nitrogen, the initial concentration of nitrogen in the lungs is 80%. The washed out nitrogen volume is, therefore, 80% of the lung volume. Thus, if the washed out nitrogen amount is 1,870 ml, the FRC volume is 2337 ml (i.e., 1,870 divided by 0.8).
A straightforward way to carry out the inert gas wash-out method for determining FRC volume is described in the Biomedical Engineering Handbook, CRC Press, 1995, ISBN 0-8493-8346-3, pp. 1237-1238. In the method therein described, the subject inhales pure oxygen and exhales into an initially empty collection reservoir until the exhaled nitrogen concentration falls below a desired concentration, for example, an end tidal value below 1%. The exhaled volume is then measured and the concentration amount of nitrogen in the exhaled volume is determined, as by means of a spectrometer. The product of the exhaled gas volume and the nitrogen concentration is the amount of nitrogen exhaled. As noted above, the latter quantity divided by the original concentration of nitrogen (0.8 in the case of air) gives the FRC volume. The foregoing determination can be expressed as ##EQU1## where F.sub.N2 is the concentration of nitrogen in the collected gas; V.sub.E is the volume of exhaled gases and 0.80 is the concentration of nitrogen in air.
However, the foregoing method, while simple, requires bulky equipment to collect tens of liters of exhaled gas. It is further necessary to ensure that there are no leaks in the system so that the patient inhales only oxygen and breathes only into the collection reservoir.
Instead of measuring the amount of inert gas which is exhaled into a collecting reservoir, the exhaled gas quantity can be obtained by integrating the product of instantaneous exhaled gas flow and the corresponding inert gas concentration. Such a measuring method is described in Crit Care Med, Vol. 18, No. 1, 1990, pp. 84-91 and in the Yearbook of Intensive Care and Emergency Medicine, Springler, 1998 (ISBN 3-540-63798-2), pp. 353-360.
This approach can be expressed as ##EQU2## where Q is exhaled gas flow, t.sub.B is the beginning time of the wash-out, and t.sub.E is the end time. The denominator of the equation notes that it is most accurate to say that the inert gas in the lungs is the difference between the beginning value F.sub.n2 (t.sub.B) and the end value F.sub.n1 (t.sub.E).
However, this technique experiences difficulties because of the need for accurate synchronization of the gas flow and concentration measurements. This may be difficult to obtain due to different delays in the different sensors for flow and for concentration. Depending on the indicator gas being used, the technique also experiences a delay caused by the wash-in of the indicator gas prior to starting the measurement. And, the measurement can be disturbed by leakages in the measurement system. This may occur, for example, with pediatric patients where unsealed intubation tubes are commonly used or when a breathing mask is imperfectly sealed to the subject. In such a case, an unmeasurable amount of the indicator gas escapes from the amount used to determine functional residual capacity.
By analogy to the above described wash-out measurement technique, it is also possible to use a wash-in of indicator gas for measurement of functional residual capacity. Such a technique and apparatus is described in European Patent Publication EP 791,327. This technique also incorporates instantaneous measurement of gas flow and corresponding indicator gas concentration and integration of the product thereof to determine indicator gas quantities. In the wash-in technique, the measurements are carried out for both inhaled and exhaled amounts of indicator gas. However, this technique, similar to the wash-out technique, experiences the difficulty of synchronization of the two measurements and a sensitivity to leakages in the system.
Further, all the foregoing techniques require the subject to be ventilated first with one gas, such as air, and then another gas, such as pure oxygen. If the subject whose functional residual capacity is being determined in this manner is a critically ill patient, variation in inspired oxygen concentration may be medically inadvisable or impossible.